Need some advice...Daisy is scratching way too much

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Started about two weeks ago. She is scratching the right side of her face to the point where her lips are swollen and scabbing 
After ruling out fleas I suspect it might just be her dirty nails that are irritating and causing her to scratch more.
Is there anything I can put on her face and lip area to disinfect and soothe? Is peroxide ok?
I feel so badly for her.
Thanks
Meredith

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Yes it's only around her mouth on one side. We just switched their food to Canidae and it's been great for them, although I never thought about food allergies. How much benadryl is good for a dog(per dose)? She is 60 lbs.
And the hydrocortizone is a great idea, I want that swelling to go down.
Thanks for the advice 

[Binky]

I love your avatar-beautiful!  Anyway, McDougal just had the same problem and it turns out it was most likely a food allergy.  We took him to the vet where he got a shot and some pills to keep the swelling and itching down.  The vet also gave us some hypo-allergenic food to use for a few weeks.  So far he is doing much better.  The vet said that a food allergy can take two weeks to a year to develop!  Hope this helps.

[Kermit]

How's Daisy doing? Did you figure out a benadryl dose? I have some itchy dogs here too... we just had a flea outbreak! In the middle of winter!! I hate to see the dogs itching.

Let us know how she's doing.

[sc.trojans]

Scratching at the mouth, chin and jaw area is usually a tell-tale sign of food allergy.  I would test another food for 30 days to see, ensuring distinctly different ingredients.  Try Nature's Variety Raw Instincts that is grain and potato free - a gluten free food like this will at least allow you to determine if it is a common gluten/grain problem.  You could also provide fresh whole foods using novel ingredients for a while and see if it improves.

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It's been awhile sorry, but I wanted to thank everyone for the great advice. Daisy is better, although I'm not 100%positive why. One of two reasons: a. we switched her from canidae to a different food...I was suspicious of the papaya, but can't be certain. and b. the dogs had been drinking from plastic bowls, which my boss(I work at an animal supply store) had informed me is a haven for bacteria. Since we switched to glass bowls and started changing her food over at the same time I have no idea what worked. But her swelling is down and she's back to normal again(yay!)